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Bumblebee learning and memory is impaired by chronic exposure to a neonicotinoid pesticide

This study explores the effects of neonicotinoid insecticides on learning and memory in bumble bees. The authors exposed bumble bees to both acute and chronic field-realistic doses of the neonicotinoid thiamethoxam and tested its effect on bumble bees on learning and memory. Bumble bees were tested for olfactory learning performance by evaluating proboscis extension reflex (PER) conditioning. As part of the acute experiment bumble bees were randomly assigned to a control group, or groups that were exposed to 2.4, 10, or 250 ppb of thiamethoxam; in the chronic exposure experiment, bumble bees were assigned to a control group, or to groups that were exposed to 2.4, or 10 ppb of thiamethoxam for 24 days. Acute exposures were intended to simulate a single bee foraging on multiple flowers from a treated crop, and chronic to simulate levels a colony would encounter foraging on a treated crop for 3-4 weeks. Bumble bees were tested for trainability (whether bees learnt the association between odor and reward, or not, over the training period), learning level (how frequently bees showed they had learned the association between odor and reward by extending their proboscis to the trained odor alone), learning speed (the first odor presentation during the training period to which a bee first showed the learned association by proboscis extension), and memory (whether bees remembered the association between odor and reward after a 3-hour break following conditioning).

In the acute exposure experiment, individual bumble bees were fed sugar water containing thiamethoxam and then tested. Bees in the control and 2.4 ppb groups were more trainable than the 250 ppb group, and control bees showed a higher learning level than the 10 and 250 ppb groups. Acute exposure to thiamethoxam did not appear to affect the learning ability or the memory of trainable.

In the chronic exposure experiment, bees were exposed to 2.4 and 10 ppb of thiamethoxam for a period of 24 days. Chronic exposure to thiamethoxam did not appear to affect trainability or learning level, but control groups had a faster learning speed response and better memory retention than treatment groups. Bees chronically exposed to thiamethoxam (at 2.4 and 10 ppb) took 27-38% more trials to learn than the controls, which could have significant impacts on foraging success and colony performance in the wild. Differences in the rates of learning seen after chronic exposure could mean that exposed bees would need to spend more time re-learning complex flowers or the location of rewarding floral patches. Treated bees (at 2.4 ppb) were less likely to respond to the stimulus after the 3-hour period post-exposure than they were at the end of the trial. The authors recommend that pesticide risk assessments should include chronic and sublethal effects on species beyond honeybees.

The results of this study show that acute exposure to thiamethoxam can affect bumble bee trainability and learning level, and that chronic exposure to thiamethoxam can affect bumble bee learning speed and memory.